Despite the best efforts, colours are not always rendered in the same way on different display media, such as a television, a computer monitor or a cinema screen. In fact, even within a particular media, there may be visible differences, such as between plasma television sets and rear projection television sets, or between different television systems or video systems.
These differences are naturally not desired by, primarily, the film and television industry, where it is desired to ensure that e.g. a film looks the same regardless of the media used for the display.
At this point, it should be pointed out that while there are several neighbouring problems—the format of the screen, the resolution of images, the rendering of the colours—the present invention is concerned with the colour problem.
A known solution to this problem, known from e.g. U.S. Pat. No. 5,604,610, uses colour processing repeatedly to produce a plurality of versions of the film, each version adapted to render colours correctly on a particular medium or media. For example, one version may be intended for cinema projection, a second for NTSC (National Television System Committee) television, and a third one for French SECAM (Séquentiel Couleur Avec Mémoire) television.
Due to the trichromatic characteristics of the human vision, colour processing most of the time uses colour triplets in various colour spaces. A basic one is RBG (for Red, Green, Blue) representing colour for TV or PC monitors. Other device dependent or device independent representations exist such as XYZ from the International Commission on Illumination (CIE).
In its strict sense—i.e. considering only colour levels and not spatial processing—colour processing realizes a colour transform for each pixel of an image, such as one film frame, or more generally for each pixel of a pixel stream.
If a, b and c are three components of an input colour triplet and a′, b′, c′ are the three components of the corresponding output triplet, the colour transform can be defined by:
      (                  a        ′            ,              b        ′            ,              c        ′              )    =            f      ⁡              (                  a          ,          b          ,          c                )              ⇔          {                                                                  a                ′                            =                                                f                  a                                ⁡                                  (                                      a                    ,                    b                    ,                    c                                    )                                                                                                                        b                ′                            =                                                f                  b                                ⁡                                  (                                      a                    ,                    b                    ,                    c                                    )                                                                                                                        c                ′                            =                                                f                  c                                ⁡                                  (                                      a                    ,                    b                    ,                    c                                    )                                                                        
While any type of colour transform can be realized in software, recent hardware developments provide hardware platforms—in the description called “colour boxes”—able to perform real-time colour transforms on high bandwidth signals such as the 2K or 4K data formats of Digital Intermediates (digital film processing). 2K corresponds to 2048×1556 colour pixels per frame at a 24 frames per second frame rate with 10 bits per colour, while 4K corresponds to four times of that (4096×3112 colour pixels).
It should be noted, however, that present day colour boxes often do not have full sets of conversion values, but that they rely on e.g. 16×16×16 control points (corresponding to 4 bit) that are expanded by tetrahedral interpolation.
It should also be noted that colour boxes may also be used to “colour grade” film sequences according to the intent of a colour grader or of the director of photography. An example is a science fiction film that takes place on a planet with a red sun. Outdoor sequences for that film could be shot in normal sunlight and processed later to make everything have a reddish tinge.
However, a problem with present day colour boxes is that they only can be used for one purpose at a given time. If a colouring artist working on his own display wishes to verify what the result looks like on the big screen, he has to load the new transform parameters into the colour box (which may take 10 seconds or more), see if it is okay, and if this is not the case, reload the original transform parameters, make a change and so on. Similarly, it is not possible to simultaneously process a plurality of versions of a scene; the colouring artist first has to work on one version before changing parameters and turning to the next.
U.S. Pat. No. 5,596,510 teaches a system for colour transformation using three look-up tables. However, the look-up tables in question—a sampled look-up table, a delta look-up table, and a restriction table—are used in series, i.e. each table is used to provide a value that is combined with the other, values to provide final transformed colour value. It will be appreciated that this does not increase the flexibility of the system as desired.
It can therefore be appreciated that there is a need for a multi-purpose apparatus for colour processor. This invention provides such a colour processor.